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Editorial Team
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Asked: 2026-05-11T16:08:18+00:00

I’m attempting to write a function in assembly that will detect if a longer

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I’m attempting to write a function in assembly that will detect if a longer binary number contains a smaller binary pattern.

Example:
Does 100111 contain 1001?

When I read this problem I figured that I would do a bitwise-AND with the large number and its smaller pattern while shifting right (logical) each time in a loop.

So, in my head I thought it would do:

100111 AND 1001 = 0  
Shift-right 1  
010011 AND 1001 = 0  
Shift-right 1  
001001 AND 1001 = 1 // Pattern FOUND!

and repeat this until either the number was shifted until it was zero or the AND returned 1.

However, I think I must have something confused because this is returning 1 for most things I put in, on the first run of the loop. Am I confused on my usage of AND?

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1 Answer

  1. Editorial Team

    The problem is that “partial matches” also return a non-zero value for your AND check:

    100111 AND 001001 = 000001

    So this tests if any of the bits match, but you want to make sure all bits are the same. The result of the AND needs to be equal to the pattern you are searching:

    x = 100111
if (x AND 1001 == 1001)
  print "found"
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